Abstract Mindfulness-based stress reduction (MBSR) successfully reduces distress and pain, and improves psychological functioning and health-related quality of life (HRQoL) among patients with chronic conditions such as cancer. MBSR interventions have also been efficacious at promoting the uptake and adherence of health behaviors. Patients living with chronic conditions such as cancer are recommended to adhere to long-term oral, anti-cancer medication regimens in order to prevent disease progression or recurrence. However, persistent side effects (e.g., fatigue, mood & sleep disturbance, vasomotor symptoms such as hot flashes, arthralgia, vaginal dryness) from oral medication regimens such as endocrine therapy (ET) for breast cancer interfere with optimal adherence to the medication. Despite the benefits of ET, documented non-adherence rates to ET range from 47% to 73%. Although there is a growing literature documenting the efficacy of MBSR on reducing the very same symptoms that interfere with optimal adherence to ET in breast cancer, no study has investigated the utility of MBSR in improving optimal adherence to life-saving oral medications via reductions in medication-related side effects and improvement in HRQoL. We have a unique opportunity to evaluate the efficacy of interventions such as MBSR in improving adherence to life-saving oral medications that have high rates of non-adherence. We propose to establish the feasibility of a web-delivered, group-based MBSR program to improve adherence to oral medication?a NCCIH high priority research topic. This is the critical first step to develop a pragmatic, scalable and accessible web-based MBSR to increase adherence to ET. Our primary outcome will be feasibility (i.e., acceptability, demand, practicality, intended limited effects) of a group, web-based MBSR program. We will also assess variability in medication adherence, ET side effects, and HRQoL as secondary outcomes as these factors may explain the effect of MBSR on adherence to ET. Our aims are 1) Develop an MBSR plus education for ET side effects condition (MBSR + ET education) and an attention-matched health plus ET education condition (health + ET education) for managing ET side effects, 2) finalize our protocol for objectively assessing adherence to ET (i.e., electronic monitoring of ET & pharmaceutical record extraction), 3) conduct usability testing with a sample of participants (N=15) on our web-delivered, group-based MBSR intervention platform to refine and finalize the web-based platform, and 4) establish the feasibility of our MBSR intervention to improve medication adherence project by implementing our study procedures and conditions (MBSR + ET education vs. health + ET education). Breast cancer survivors (N = 80) will be randomized to the conditions (MBSR + ET education n = 40; health + ET education n = 40). We hypothesize that the MBSR + ET education will have higher feasibility outcomes (i.e., acceptability, demand, practicality, limited efficacy) than the control condition due to the inclusionof MBSR self-management content and training skills.